Investigation of Stresses in Aluminium A356 Alloy Pelton Turbine Buckets

Abstract

Rapid increase in energy consumption and global warming threatening the environment together with the unpredictable increases of the fossil fuel prices has increased the importance of renewable energy sources. The need to reduce use of fossil fuel as energy sources in rural areas which are not connected to power grid is a major concern in order to stop a further decline in the environment. To replace this nonrenewable energy many different alternative power sources are being researched on and implemented. Demand for rural electrification in the country has triggered use of small scale hydro turbines. Therefore, manufacturing of turbines using locally available materials such as recycled aluminium scrap metal has resulted. Effective performance of these turbines, turbine efficiency and dynamic behavior under different operating conditions need to be predicted. This research investigates the variation on ratio between the bucket width and the jet diameter on turbine performance. A decrease in the ratio leads to an increase in cyclic load and therefore an increase in stresses on the buckets and the turbine hub. This is a notable problem with bolted runner Pelton buckets. In addition to cyclic loading, corrosion aggravates stress concentrations around the bolts leading to turbine failure. This work sought to improve the profile of Pelton turbine buckets in order to optimize their performance. This was accomplished through design, manufacture and testing of improved models of the buckets. Design and production of the test Pelton buckets was done using Computer Aided Design (CAD) and Computer Aided Manufacturing (CAM) software. Investigation of forces and stresses on a Pelton buckets was done using structural analysis on commercial simulation software, ANSYS Structural. The simulation results were then validated with experimental results obtained from bending test conducted on the two buckets samples. The investigation indicated a 14% percentage reduction on the stresses developed on the buckets by introducing a ridge on the backside of the bucket that enabled even distribution of stresses on the bucket without compromise on the bucket width to jet diameter ratio. As a result, without altering the optimum bucket width in relation to water jet diameter a higher power output was achieved by optimizing the bucket profile.